A steel for the main bearing of a shield machine with high cleanliness and high carbon equivalent for the production of continuous casting large round billets and its manufacturing method

Abstract

The present invention relates to a high-purity and high-carbon-equivalent steel for shield machine main bearings used in continuous casting of large round billets and a manufacturing method thereof. The steel for bearings is based on Fe as the basic element and contains the following chemical components in mass percentages: C: 0.40% - 0.46%, Si: 0.20% - 0.40%, Mn: 0.80% - 0.90%, P ≤ 0.015%, S ≤ 0.01%, Cr: 1.00% - 1.20%, Ni: 0.40% - 0.60%, Mo: 0.20% - 0.30%, Cu ≤ 0.20%, Al: 0.015% - 0.04%, Ti ≤ 0.002%, Nb ≤ 0.003%, V ≤ 0.01%, Ca ≤ 0.001%, H ≤ 0.0015%, O ≤ 0.001%, N: ≤ 0.008%, As ≤ 0.01%, Sn ≤ 0.004%, Sb ≤ 0.005%, Pb ≤ 0.003%, Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 ≥ 0.85%, and the balance is Fe and inevitable impurities. The product manufactured in this application improves the carbon equivalent due to the innovation in composition design and the cleanliness due to the innovation in casting process compared with traditional products, making the steel have better strength and toughness, more reasonable fatigue resistance and mechanical properties, higher production efficiency, lower cost, and achieving good effects of improving quality and reducing costs.

Description

Technical Field

[0001] The present invention belongs to the technical field of special steel smelting, and particularly relates to a high-cleanliness, high-carbon-equivalent steel for shield machine main bearings produced by continuous casting of large round billets and a manufacturing method thereof. Background Art

[0002] A shield machine is the main construction machinery in underground tunneling shield construction. While preventing the collapse of the soft foundation excavation surface or maintaining the stability of the excavation surface, it can safely carry out tunnel excavation and lining operations inside the machine. Currently, it has been widely used in large tunnel engineering projects such as urban subways, cross-river highway tunnels, and cross-sea tunnels in China. The main bearing of the shield machine plays a role in supporting the cutter head of the shield machine and rotating it to break rocks. Therefore, the main bearing of the shield machine is a key and core component of the shield machine, regarded as the heart of the shield machine. For a long time, it has been relying on imports. Foreign products have long-term technical monopolies, high prices, long delivery periods, and at the same time, the service quality cannot meet the requirements of shield machine manufacturing and construction enterprises. There are more than 2,000 shield machines in China, and more than 400 are newly added every year. And the bearings need to be maintained and replaced regularly. With the full spread of shield tunneling construction in urban subways, railway tunnels, highway tunnels, water conservancy projects, urban municipal gas pipeline projects, sewage pipeline projects, heating and cooling pipeline projects, and cable pipeline projects in China, the total market demand for shield machines shows a rapid growth trend, bringing a broad market for shield machine bearings, and the demand for the localization of shield machine bearings is becoming increasingly urgent. Therefore, the localization of the steel for shield machine main bearings has become the primary task of solving the problem, which is of great significance for improving the level of China's equipment manufacturing and the core competitiveness of major equipment and maintaining the national economic security. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide a high-cleanliness, high-carbon-equivalent steel for shield machine main bearings produced by continuous casting of large round billets and a manufacturing method thereof according to the above-mentioned prior art, so as to meet the axial bearing capacity of the greater load borne by tunneling construction under rock geological conditions and further improve the strength and hardness of the bearing.

[0004] The technical solution adopted by the present invention to solve the above problems is as follows: A high-cleanliness and high-carbon-equivalent steel for shield machine main bearings produced by continuous casting of large round billets. The steel for the main bearing is based on Fe as the basic element and contains the following chemical components in mass percentages: C: 0.40% - 0.46%, Si: 0.20% - 0.40%, Mn: 0.80% - 0.90%, P ≤ 0.015%, S ≤ 0.01%, Cr: 1.00% - 1.20%, Ni: 0.40% - 0.60%, Mo: 0.20% - 0.30%, Cu ≤ 0.20%, Al: 0.015% - 0.04%, Ti ≤ 0.002%, Nb ≤ 0.003%, V ≤ 0.01%, Ca ≤ 0.001%, H ≤ 0.0015%, O ≤ 0.001%, N: ≤ 0.008%, As ≤ 0.01%, Sn ≤ 0.004%, Sb ≤ 0.005%, Pb ≤ 0.003%, Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 ≥ 0.85%, and the balance is Fe and unavoidable impurities.

[0005] The non-metallic inclusions of the steel for the main bearing are inspected according to the requirements of Method A of GB / T 10561 standard and meet the provisions of the following table.

[0006] Qualified level of non-metallic inclusions (grade)

[0007]

[0008] The mechanical properties of the steel for the main bearing should meet the performance requirements of tensile strength ≥ 930 (Mpa), yield strength ≥ 820 (Mpa), reduction of area A ≥ 50%, elongation ≥ 13%, average impact energy (KV2) at -20°C ≥ 70J, and single value ≥ 49J.

[0009] The internal quality of the steel for the main bearing should meet the following flaw detection requirements: After turning and flaw detection, it meets the flaw detection requirements of Grade I in JB / T5000.15 standard (single defect ≤ Ф2mm). The grain size meets: For ring forgings (forging ratio ≥ 5), after normalizing and quenching and tempering, the sample is taken from the body and evaluated according to GB / T6394, and the average austenite grain size ≥ 7 grades.

[0010] The functions and dosage selections of the main element components contained in the present application are specifically described as follows:

[0011] 1) Determination of C content

[0012] C is the main element that ensures the hardness and strength of steel and also significantly improves the hardenability of steel. However, too high a C content in steel will reduce the plasticity and toughness of the material. According to the load-bearing requirements of the cutter head of the shield machine, in order to achieve higher strength and high toughness requirements, this application adopts a medium-carbon solution, controlling its C content to be 0.40% - 0.46%, which is increased compared to the ordinary medium-carbon bearing steel 42CrMo.

[0013] 2) Determination of Si content

[0014] Si is a deoxidizing element in steel and increases the hardness and strength of steel in the form of solid solution strengthening. When the Si content is lower than 0.15%, the deoxidation effect of the steel is poor, and when the Si content is relatively high, the toughness is reduced. The Si content in this application is controlled to be 0.20 - 0.40%.

[0015] 3) Determination of Mn content

[0016] Mn is an element that improves the hardenability of steel and also plays a role in solid solution strengthening. It also forms long-strip MnS with S, reducing the harmfulness of S. The Mn content in this application is controlled at 0.80 - 0.90%, which is increased compared to the ordinary medium-carbon bearing steel 42CrMo.

[0017] 4) Determination of Cr content

[0018] Cr is an element that improves the hardenability of steel and also has the effect of improving impact toughness. Considering the large-scale of the main bearing of the shield machine, it is necessary to improve the overall quenching and tempering performance of the entire cross-section. Therefore, it is appropriately increased compared to the ordinary medium-carbon bearing steel 42CrMo. The Cr content in this application is controlled at 1.00 - 1.20%.

[0019] 5) Determination of Mo content

[0020] Mo is an element that efficiently improves the hardenability of steel and has an ideal effect on improving the strength and toughness of steel, but it is expensive. The Mo content in this application is controlled at 0.20 - 0.30%.

[0021] 6) Determination of Ni content

[0022] Ni is an element that improves the hardenability of steel and has an obvious effect on improving the impact toughness of steel. This is very important for improving the strength and toughness of thick workpieces, which is not added in the ordinary medium-carbon bearing steel 42CrMo. The Ni content in this application is specifically controlled at 0.40 - 0.60%.

[0023] 7) Determination of Al content

[0024] Al is a strong deoxidizing element and also a grain-refining element. Appropriate addition can achieve good metallurgical and fine-grain effects, but excessive addition is likely to form alumina inclusions, so it can only be added in trace amounts. The content in this application is controlled at 0.015 - 0.04%.

[0025] 8) Determination of P and S Contents

[0026] P and S are harmful impurity elements in steel. They are prone to form segregation, inclusions and other defects, and are also harmful to the impact toughness of steel. Their contents should be minimized as much as possible under the condition of controllable cost. In this application, P≤0.015% and S≤0.01% are controlled.

[0027] 9) Determination of As, Sn, Sb, Pb Contents

[0028] As, Sn, Sb, Pb, and Bi are low-melting-point impurity elements. When they exist in steel, they are likely to cause soft spots and uneven hardness on the surface of parts. Therefore, they are regarded as harmful elements in steel. In this application, the content ranges of these elements are determined as As≤0.01%, Sn≤0.004%, Sb≤0.005%, and Pb≤0.003%.

[0029] 10) Determination of H, O, N Contents

[0030] The oxygen content represents the total amount of oxide inclusions. Oxide brittle inclusions reduce the impact toughness of steel; in terms of the H content, the delayed cracks caused by hydrogen affect the flaw detection performance and impact toughness of the product. Therefore, it is necessary to strengthen the vacuum degassing control during the smelting process. N is likely to form sharp-angle hard TiN inclusions in steel and should be controlled as much as possible. In this application, finally, H≤0.00015%, O≤0.001%, and N≤0.008% are controlled.

[0031] 11) Setting of Carbon Equivalent

[0032] Considering the high-load longitudinal bearing capacity of the main bearing of the shield machine and the high hardenability and comprehensive performance required for large-scale applications, this application specifically sets a high control for the carbon equivalent, so that the comprehensive performance of the product is finally improved compared with that of the ordinary medium-carbon bearing steel 42CrMo. Therefore, Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15≥0.85% is set.

[0033] 12) Other Residual Elements

[0034] Ti and Ca are prone to form endogenous inclusions, reducing the fatigue life of the bearing, so they are strictly controlled; Nb and V can refine the grain size, but they affect the hardenability and they are both precious metals with high costs, so they are controlled as much as possible. Therefore, in this application, Ti≤0.002%, Ca≤0.001%, Nb≤0.003%, and V≤0.01% are controlled.

[0035] The steel billets used for the main bearings of traditional shield machines are all produced by the conical die casting method or the electroslag remelting method. These methods generally have high energy consumption, low efficiency, and high costs, and are not suitable for the current world's need for green and low-carbon development. Moreover, it is difficult to control the cleanliness and quality stability of die casting, and it is difficult to meet the requirements of high fatigue life and low cost of shield machine bearings under solid rock geological conditions. Therefore, this application innovatively uses large round billet continuous casting to replace die casting or electroslag production of bearing steel billets in this field. The continuous casting process can obtain relatively cleaner molten steel through multi-stage filtration of inclusions in the molten steel. In addition, the cylindrical billet is more "near-net shape" than the conical ingot when forging the bearing ring, and it can also reduce the downstream forging process. Therefore, large round billet continuous casting can improve the cleanliness of molten steel while improving efficiency and reducing costs, realizing green manufacturing.

[0036] The present invention provides a manufacturing method for a high-cleanliness and high-carbon equivalent steel for the main bearing of a shield machine produced by continuous casting of large round billets, including KR hot metal pretreatment - BOF converter smelting - LF refining - RH vacuum degassing - Round billet continuous casting (equipped with three-stage electromagnetic stirring) - slow cooling - heating forging and ring rolling - slow cooling - normalizing - quenching and tempering treatment - turning - flaw detection - inspection and warehousing, which is realized through the following manufacturing process:

[0037] Blank manufacturing: After the hot metal is treated by KR, it meets the requirements of Si≤0.1%, S≤0.01%. Then, it is mixed with scrap steel materials in a ratio of more than about 4:1 to prepare the smelting raw materials, and then added to the converter for smelting. The converter should ensure that the P content in the molten steel is ≤0.010% when tapping, and at the same time, C≥0.10% to prevent the molten steel from being over-oxidized. At the same time, it is ensured that Sn, As, Sb, and Pb all meet the requirements. An alloy baking oven should be equipped behind the converter. All alloys added during tapping should be baked in the oven as much as possible to remove moisture. When tapping from the converter, a slag stopper or slag ball should be used to block the slag to prevent oxidized slag from entering the LF refining process and causing P return and oxide inclusions. The molten steel enters the LF refining process for slag making, stirring, deoxidation, and alloying. It is necessary to ensure good deoxidation of the molten steel. Finally, Al wire is fed for final deoxidation to ensure that the total [O] in the steel is ≤10ppm. Then, the chemical composition is adjusted by mass percentage to C: 0.40% - 0.46%, Si: 0.20% - 0.40%, Mn: 0.80% - 0.90%, P≤0.015%, S≤0.01%, Cr: 1.00% - 1.20%, Ni: 0.40% - 0.60%, Mo: 0.20% - 0.30%, Cu≤0.20%, Al: 0.015% - 0.04%, Ti≤0.002%, Nb≤0.003%, V≤0.01%, Ca≤0.001%, H≤0.0015%, O≤0.001%, N: ≤0.008%, As≤0.01%, Sn≤0.004%, Sb≤0.005%, Pb≤0.003%, Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15≥0.85%. At the same time, the argon stirring should be sufficient to make the solid white slag float out of the molten steel and enter the slag for removal, achieving the effect of purifying and refining the molten steel. The RH vacuum refining should ensure that the vacuum degree in the RH vacuum chamber is ≤67Pa, the vacuum degassing time is greater than 20min. After vacuum treatment, the molten steel can reach the following gas analysis levels: H≤1.5ppm, Otot≤10ppm, N≤80ppm. The refined molten steel is sent to the arc-shaped continuous casting machine for pouring, poured into a round billet with a diameter of ≥φ1000mm. The liquid level fluctuation during pouring in the mold is controlled within ±5mm. Mold electromagnetic stirring: frequency is (0 - 3)Hz, current is (1 - 250)A; secondary cooling section stirring: frequency is (0 - 3)Hz, current is (1 - 250)A; final electromagnetic stirring: frequency is (0 - 4)Hz, current is (1 - 350)A. Finally, a continuous casting round billet with a diameter of ≥φ1000mm and high cleanliness molten steel is produced, which is dense and homogeneous (the chemical composition, inclusions, gases, etc. in the steel all meet the design requirements). The continuous casting round billet is cut into segments according to the customer's specified length by flame cutting and then slowly cooled in a pit at a temperature above 550℃ for more than 96h, and then taken out of the pit for finishing and warehousing. The innovation of this application lies in: using a continuous casting round billet with a diameter of ≥φ1000mm to replace the ingot casting or electroslag ingot, improving quality, efficiency, and reducing costs.

[0038] Forging: Blanks with qualified composition and macrostructure of φ1000mm or φ1200mm specifications are sawed and cut according to the required weight of the workpiece, and then sent to a heating furnace for heating to 1200 - 1210°C. The total heating time is more than 20 hours. Then, it is taken out of the furnace and sent to an 8000-ton hydraulic press for three-upsetting and two-drawing to make the billet, ensuring that the porosity and shrinkage cavity in the center of the continuous casting billet are fully closed. Then, punching is carried out (punch diameter φ700). After punching, it is put into the furnace for secondary heating, heated to 1200 - 1210°C again, and then ring rolling is carried out (using a ring rolling machine with a maximum diameter of 10m). The final temperature of ring rolling is ≥860°C. The ring rolling ratio ≥3.0 (the ring rolling ratio refers to the area ratio of the cross-section of the ring before and after ring rolling), thus ensuring the flaw detection requirements of large bearing blanks. After forging, it is placed in a sand pit for stacking and slowly cooled to room temperature, which can prevent the generation of white spots and avoid surface stress cracks caused by rapid cooling.

[0039] Heat treatment: This application adopts the "normalizing + quenching and tempering" heat treatment process. After the slow cooling of the forged blank, normalizing is carried out first. The normalizing temperature is 880±10°C, and it is air-cooled to room temperature after being taken out of the furnace to facilitate uniform refinement of the grain structure. Then, it is reheated and quenched. The quenching heating temperature is 865±10°C, and the holding time depends on the thickness of the workpiece: calculated as (1.5 - 2.0 min / mm). Quenching is carried out using quenching pool water until the surface temperature of the workpiece is lower than 100°C, and then it is air-cooled to room temperature. Then, high-temperature tempering is carried out. The tempering heating temperature is 590±10°C, and the holding time is calculated as (2.5 - 3.5 min / mm), and it is air-cooled to room temperature after being taken out of the furnace.

[0040] Sampling and testing: Samples are taken from the bearing sleeve blank after heat treatment at a depth of 12.5mm from the surface for performance and inclusion testing, meeting the performance requirements of tensile strength ≥930 (Mpa), yield strength ≥820 (Mpa), reduction of area A ≥50%, elongation ≥13%, average impact energy at -20°C (KV2) ≥70J, and individual ≥49J. After turning, flaw detection is carried out, meeting the flaw detection requirements of Grade I in JB / T5000.15 standard (single defect ≤Ф2mm). Inclusions meet the high-level requirements in the following table:

[0041]

[0042]

[0043] Compared with the prior art, the advantages of the present invention are as follows:

[0044] 1. High carbon equivalent design. To meet the increasing axial bearing capacity of the larger loads borne by tunneling construction in more and more rock geological conditions and further improve the strength and hardness of the bearing, on the basis of the original 42CrMo composition, the high carbon equivalent is controlled to meet Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 ≥ 0.85%;

[0045] 2. High-toughness design: When the carbon equivalent is increased to meet the requirements of increased strength and hardness of the bearing, the toughness is reduced, and there is a great risk of quenching and tempering heat treatment cracks. Therefore, in this application, Ni element is innovatively added and controlled between 0.40 - 0.60%, ensuring sufficient hardenability depth of large forgings of bearings weighing more than ten tons and dozens of tons, while also obtaining uniform and stable good impact toughness, achieving good strength-toughness matching.

[0046] 3. Change the traditional process of forging shield machine main bearings using ingot casting or electroslag ingots at home and abroad. Instead, use ultra-large-sized continuous casting round billets as the mother material for forging. This improves the cleanliness of the steel, simplifies the forging process, and greatly improves the forging efficiency, achieving comprehensive effects of improving quality, reducing costs, and increasing efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 It is the metallographic structure diagram of Embodiment 1 of the present invention.

[0048] Figure 2 It is the metallographic structure diagram of Embodiment 2 of the present invention.

[0049] Figure 3 It is the metallographic structure diagram of Embodiment 3 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0050] The technical solution of the present invention will be described in more detail in combination with the preferred embodiments of the present invention. However, these embodiments are only descriptions of the preferred embodiments of the present invention and cannot limit the scope of the present invention in any way.

[0051] Embodiments 1 - 3 respectively give examples of the chemical composition and manufacturing method of the high-purity and high-carbon equivalent steel for shield machine main bearings of this application, and compare them with the traditional shield machine main bearing steel 42CrMo steel.

[0052] The chemical composition of each embodiment is shown in Table 1.

[0053] Table 1 wt%

[0054]

[0055]

[0056] Continued Table 1 wt%

[0057]

[0058] The inclusions of the steel of each embodiment are shown in Table 2.

[0059] Table 2

[0060] The purity of the examples is significantly better than that of traditional products.

[0061] The comparison of the mechanical properties of each example is shown in Table 3.

[0062] Table 3

[0063] The examples fully meet the performance requirements of cross-sectional shrinkage rate A≥50%, elongation rate≥13%, average impact energy (KV2) at -20°C≥70J, and single value≥49J, which are difficult to achieve for traditional products.

[0064] 12 bearing rings are manufactured from the billets of each example, and all of them are qualified in flaw detection according to the flaw detection requirements of Grade I in JB / T5000.15 standard (single defect≤Ф2mm), and no excessive defects are found. However, there is still a UT defective rate of 3-5% for the usually continuous casting products.

[0065] The microstructure of each example is a uniform and fine ferrite + pearlite structure, and the grain size is between 7-10 grades, as shown in Appendix Figure 1 、 2 、3.

[0066] From the above test results, it can be seen that the performance of each example is better than that of traditional products.

[0067] The manufacturing process of the steel for the main bearing of the shield machine in each example is: KR hot metal pretreatment - BOF converter - LF refining - RH vacuum degassing - round billet continuous casting - slow cooling - heating for forging and ring rolling - slow cooling - normalizing - quenching and tempering - turning - flaw detection - inspection and warehousing.

[0068] During specific smelting, high-quality hot metal, scrap steel, and raw and auxiliary materials with low P and low S are selected, as well as high-quality deoxidizers and refractory materials. The BOF converter controls the end-point C≥0.10%, tapping temperature≥1620°C, the LF refining has good deoxidation, the white slag holding time≥30min, and the total smelting time is 50 - 60min; the RH high vacuum (≤67Pa) time≥20min, and the net circulation time≥15min; the continuous casting superheat is controlled within 25 - 40°C. In Example 1 and Example 2, a Ф1000mm round billet is used (for producing a Ф4382*4120*205 bearing ring), and the continuous casting casting speed is controlled at 0.11 - 0.13m / min. In Example 3, a Ф1200mm round billet is used (for producing a Ф5095*4720*210 bearing ring), and the continuous casting casting speed is controlled at 0.09 - 0.10m / min. During continuous casting, three-stage electromagnetic stirring is adopted. The mold electromagnetic stirring: the frequency is 0.5Hz and the current is 130A; the secondary cooling section stirring: the frequency is 2Hz and the current is 70A; the final electromagnetic stirring: the frequency is 2Hz and the current is 200A. After continuous casting, it is flame cut and segmented according to the user's required cut length. The cast billet is slowly cooled in a pit at a temperature of 550°C - 560°C, and the slow cooling time is more than 96h. Then it is sent to the heating furnace of the forging factory and heated to 1200 - 1210°C, and the total heating time is 20 - 25 hours to make the steel billet heated evenly and thoroughly, ensuring the composition and performance of the final product. The forging start temperature is 1100 - 1120°C, and the final forging temperature is 860°C - 870°C. The three-upsetting-two-drawing - punching - ring rolling process is adopted, and the total forging ratio is 6 - 8. After forging, it is placed in a sand pit for slow cooling or stacked for cooling until room temperature. There are no crack defects on the surface after slow cooling. Then normalizing is carried out. The normalizing temperature in Example 1 and Example 2 is 880°C, and in Example 3 is 886°C. It is air-cooled to room temperature after being taken out of the furnace. Then it is reheated and quenched. The quenching heating temperature in Example 1 and Example 2 is 872°C, and the holding time is 4h. In Example 3, it is 878°C, and the holding time is 5.6h. Quenching is carried out using quenching pool water quenching. After the surface temperature of the workpiece is cooled below 100°C, it is air-cooled to room temperature. Then high-temperature tempering is carried out. The tempering heating temperature in Example 1 and Example 2 is 595°C, and the holding time is 6.5h. In Example 3, it is 600°C, and the holding time is 9.5h. It is air-cooled to room temperature after being taken out of the furnace.

[0069] As can be seen from Tables 1, 2, and 3, compared with the traditional 42CrMo steel, a high-purity and high-carbon equivalent steel for shield machine main bearings in each of the above embodiments of the present application, through the optimization of elemental composition and the innovative design of the casting process. After being treated by the same forging and heat treatment process, the purity, yield strength, tensile strength, and low-temperature impact value of the present application are significantly better than those of the traditional 42CrMo steel. Especially, the strength and toughness and the UT flaw detection qualification rate are better, and various performance indicators are more stable, indicating that the effect of the present invention is ideal.

[0070] Although the preferred embodiments of the present invention have been described in detail above, it should be clearly understood that various modifications and variations can be made to the present invention by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A manufacturing method of steel for the main bearing of a shield machine with high cleanliness and high carbon equivalent for continuous casting of large round billets, characterized in that The steel for the main bearing is based on Fe and contains the following chemical components by mass percentage: C: 0.40% - 0.46%, Si: 0.20% - 0.40%, Mn: 0.80% - 0.90%, P ≤ 0.015%, S ≤ 0.01%, Cr: 1.00% - 1.20%, Ni: 0.40% - 0.60%, Mo: 0.20% - 0.30%, Cu ≤ 0.20%, Al: 0.015% - 0.04%, Ti ≤ 0.002%, Nb ≤ 0.003%, V ≤ 0.01%, Ca ≤ 0.001%, H ≤ 0.0015%, O ≤ 0.001%, N: ≤ 0.008%, As ≤ 0.01%, Sn ≤ 0.004%, Sb ≤ 0.005%, Pb ≤ 0.003%, Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 ≥ 0.85%, and the balance is Fe and inevitable impurities. The method includes the following steps: 1) Blank manufacturing: After the hot metal is treated by KR, it meets the requirements of Si ≤ 0.1% and S ≤ 0.01%. Then, it is formulated with scrap steel materials in a ratio of more than 4:1 to prepare the smelting raw materials, and then added to the converter for smelting. The converter should ensure that the tapping P ≤ 0.010%, and at the same time C ≥ 0.10% to prevent the molten steel from being over-oxidized. At the same time, it is ensured that Sn, As, Sb, and Pb all meet the requirements. After the converter, the molten steel enters the LF refining for slag making, stirring, deoxidation, and alloying to ensure good deoxidation of the molten steel. Finally, Al wire is fed for final deoxidation to ensure that the total [O] in the steel ≤ 10 ppm; then the chemical components are adjusted to the target values by mass percentage. At the same time, the argon stirring should be sufficient to make the solid white slag float out of the molten steel and enter the slag to be removed, achieving the effect of purifying and purifying the molten steel. After RH vacuum refining, the molten steel is sent to the arc continuous caster for pouring, and poured into a round billet with a diameter ≥ φ1000mm; 2) Forging: The billets with qualified composition and macrostructure of the specification ≥ φ1000mm are sawed and cut according to the weight required for the workpiece, and then heated in a heating furnace to 1200 - 1210°C for a total heating time of more than 20 hours. Then, it is taken out of the furnace and sent to a hydraulic press for three upsetting and two drawing to make the billet to ensure that the central porosity and shrinkage cavity of the continuous casting billet are fully closed. Then, punching is carried out. After punching, it is reheated in the furnace, reheated to 1200 - 1210°C again, and then ring rolling is carried out. The final temperature of ring rolling ≥ 860°C; 3) Heat treatment: Adopt the "normalizing + quenching and tempering" heat treatment process. After the forging blank is slowly cooled, it is first normalized. The normalizing temperature is 880 ± 10°C, and it is air-cooled to room temperature after being taken out of the furnace to facilitate uniform refinement of the grain structure. Then, it is reheated and quenched. The quenching heating temperature is 865 ± 10°C, and the holding time depends on the thickness of the workpiece: carried out at a rate of 1.5 - 2.0 min / mm. Quenching is carried out using quenching pool water quenching. After the surface temperature of the workpiece is quenched below 100°C, it is air-cooled to room temperature, and then high-temperature tempering is carried out. The tempering heating temperature is 590 ± 10°C, and the holding time is 2.5 - 3.5 min / mm. It is air-cooled to room temperature after being taken out of the furnace; 4) Sampling and testing: Samples are taken from the bearing sleeve blank after heat treatment at a depth of 12.5 mm from the surface for performance and inclusion testing. The obtained performance meets the requirements of tensile strength ≥930 Mpa, yield strength ≥820 Mpa, reduction of area A ≥50%, elongation ≥13%, average impact energy KV2 at -20°C ≥70 J, and single impact energy ≥49 J. After turning, flaw detection is carried out, meeting the flaw detection requirements of Grade I in JB / T5000.15 standard.

2. The manufacturing method of the steel for the main bearing of a shield machine with high cleanliness and high carbon equivalent used in the production of continuously cast large round billets according to claim 1, characterized in that: The non-metallic inclusions of the steel for main bearings are inspected according to the requirements of Method A in GB / T10561 standard, meeting A fine series ≤1.0, A coarse series ≤1.0, B fine series ≤1.0, B coarse series ≤1.0, C fine series ≤0.5, C coarse series ≤0.5, D fine series ≤1.0, D coarse series ≤1.0, Ds fine and coarse series ≤1.

5. The evaluation is carried out according to GB / T6394, and the average austenite grain size ≥7 grades.

3. The manufacturing method of the steel for the main bearing of a shield machine with high cleanliness and high carbon equivalent for continuous casting of large round billets according to claim 1, characterized in that: The steel for main bearings meets the performance requirements of tensile strength ≥930 Mpa, yield strength ≥820 Mpa, reduction of area A ≥50%, elongation ≥13%, average impact energy KV2 at -20°C ≥70 J, and single impact energy ≥49 J.

4. The manufacturing method of the steel for the main bearing of a shield machine with high cleanliness and high carbon equivalent for continuous casting of large round billets according to claim 1, characterized in that Alloy baking ovens are equipped behind the converter. The alloys added during tapping are pre-baked in the oven until they are in place to remove moisture. Slag stopper or slag ball is used for slagging during converter tapping to prevent oxidized slag from entering the LF refining process and avoid re-introduction of P and generation of oxide inclusions.

5. The manufacturing method of the steel for the main bearing of a shield machine with high cleanliness and high carbon equivalent for continuous casting of large round billets according to claim 1, characterized in that RH vacuum refining ensures that the vacuum degree in the RH vacuum tank ≤67 Pa, the vacuum degassing time is greater than 20 min. For the molten steel after vacuum treatment, the gas analysis reaches the following levels: H ≤1.5 ppm, Otot ≤10 ppm, N ≤80 ppm.

6. The manufacturing method of the steel for the main bearing of a shield machine with high cleanliness and high carbon equivalent for the production of continuously cast large round billets according to claim 1, characterized in that During the pouring of molten steel, three-stage electromagnetic stirring technology is adopted. The liquid level fluctuation in the mold during pouring is controlled within ±5 mm. Mold electromagnetic stirring: frequency is 0 - 3 Hz, current is 1 - 250 A; secondary cooling section stirring: frequency is 0 - 3 Hz, current is 1 - 250 A; final electromagnetic stirring: frequency is 0 - 4 Hz, current is 1 - 350 A.

7. The manufacturing method of the steel for the main bearing of a shield machine with high cleanliness and high carbon equivalent for continuous casting of large round billets according to claim 1, characterized in that The continuous casting round billets are cut into segments according to the specified length by flame cutting and then slow-cooled in a pit at a temperature above 550°C for more than 96 h, and then taken out of the pit for finishing and warehousing.

8. The manufacturing method of the steel for the main bearing of a shield machine with high cleanliness and high carbon equivalent used in the production of continuously cast large round billets according to claim 1, characterized in that During the forging process, the forging ratio of the ring is ≥3.

0. After forging, it is placed in a sand pit and stacked for slow cooling to room temperature.

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